The present invention relates to a system for measuring the velocity of a moving object in relation to a fixed object such as the ground or sea surface of the seabed. It relates particularly to a system which employs the Doppler effect to measure the velocity of the moving object on board which it is situated as projected onto one or more reference axes.
Known systems of this type, such as Doppler radar systems for serial navigation, transmit electromagnetic waves towards the ground and measure the difference in frequency between the transmitted waves and those reflected by the ground. This difference in frequency is proportional to the velocity of the moving object. However, the spectrum of the ground echoes is spread over a certain frequency range. This frequency range becomes the wider the greater the angular extent of the main lobe of the antenna of the radar is and the farther away the line of the direction of the antenna is from that of the velocity vector of the moving object. To ensure that measurement is not excessively innaccurate it is necessary to use a very directional, and therefore large, heavy and cumbersome, antenna or antennas which give a very fine lobe.
The measuring system according to my invention does not have these drawbacks. It measures the velocity of the moving object on the basis of the difference in phase of the signal reflected by the ground at two different instants and, to give increased accuracy, it uses at the input to the measuring circuits signals which are a linear combination of the sum and difference signals from a monopulse receiver, this being done to bring about an artificial shift in the phase center of the antenna.